diff --git a/models/rfd3/src/rfd3/inference/input_parsing.py b/models/rfd3/src/rfd3/inference/input_parsing.py
index d97b3be3..3d922466 100644
--- a/models/rfd3/src/rfd3/inference/input_parsing.py
+++ b/models/rfd3/src/rfd3/inference/input_parsing.py
@@ -136,6 +136,7 @@ class DesignInputSpecification(BaseModel):
     # Extra args:
     length:  Optional[str] = Field(None, description="Length range as 'min-max' or int. Constrains length of contig if provided")
     ligand:  Optional[str] = Field(None, description="Ligand name or index to include in design.")
+    allow_ligand_on_existing_chain: bool = Field(False, description="If True, suppress the error when a ligand shares a chain ID with the built atom array. Use with caution — chain ID is leaked to the model.")
     cif_parser_args: Optional[Dict[str, Any]] = Field(None, description="CIF parser arguments")
     extra: Optional[Dict[str, Any]] = Field(default_factory=dict, description="Extra metadata to include in output (useful for logging additional info in metadata)")
     dialect: int = Field(2, description="RFdiffusion3 input dialect. 1: legacy, 2: release.")
@@ -672,14 +673,45 @@ def _append_ligand(self, atom_array, atom_array_input_annotated):
                     + list(atom_array_input_annotated.get_annotation_categories())
                 ),
             )
-            # Offset ligand residue ids based on the original input to avoid clashes
-            # with any newly created residues (matches legacy behaviour).
-            ligand_array.res_id = (
-                ligand_array.res_id
-                - np.min(ligand_array.res_id)
-                + np.max(atom_array.res_id)
-                + 1
-            )
+            # Validate chain assignments — chain ID is leaked to the model
+            # so collisions are a significant deviation from convention.
+            ligand_chains = np.unique(ligand_array.chain_id)
+            existing_chains = set(np.unique(atom_array.chain_id))
+            overlapping = sorted(existing_chains & set(ligand_chains))
+            if not self.allow_ligand_on_existing_chain:
+                if overlapping:
+                    raise ValueError(
+                        f"Ligand chain(s) {overlapping} overlap with existing "
+                        f"chain(s) {sorted(existing_chains)}. Place ligands on "
+                        f"separate chains or set 'allow_ligand_on_existing_chain: "
+                        f"true' to restore the old behaviour."
+                    )
+                # Multiple ligands must each be on their own chain.
+                for chain in ligand_chains:
+                    n_residues = len(
+                        np.unique(ligand_array.res_id[ligand_array.chain_id == chain])
+                    )
+                    if n_residues > 1:
+                        raise ValueError(
+                            f"Multiple ligand residues on chain {chain}. Each "
+                            f"ligand must be on its own chain, or set "
+                            f"'allow_ligand_on_existing_chain: true' to restore "
+                            f"the old behaviour."
+                        )
+            if self.allow_ligand_on_existing_chain:
+                # Legacy behaviour: offset from protein max to avoid clashes.
+                ligand_array.res_id = (
+                    ligand_array.res_id
+                    - np.min(ligand_array.res_id)
+                    + np.max(atom_array.res_id)
+                    + 1
+                )
+            else:
+                # Reset ligand res_id to start from 1 per chain, matching
+                # the convention AF3 uses in its output CIF files.
+                for chain in ligand_chains:
+                    mask = ligand_array.chain_id == chain
+                    ligand_array.res_id[mask] = 1
             # Harmonize conditioning annotations before concatenation: biotite's
             # concatenate only preserves annotations present in ALL arrays (set
             # intersection), so mismatched optional conditioning annotations